The strongest single factor seems to be family history. This means that acne has a genetic basis.
Hormonal changes in adolescence: Stimulation of the sebaceous glands occurs with the production of androgens (the masculinizing hormone found in both sexes) at puberty.
Acne patients seem to produce normal amounts of androgen, but their skin is unusually sensitive to its production.
Skin flora, that is the kind of bacteria that are normally found on a person's skin.
External irritants, like creams,excessive exposure to the sun-rays, etc.
Nutrition. High fat food.

Factors that cause Acne:

Normal increase in the levels of the hormone androgen during adolescence.
Changes in hormone levels before a woman's menstrual period or during pregnancy.
Rich moisturizing lotions or heavy or greasy makeup.
Emotional stress.
Nutritional supplements that have iodine.
Putting pressure on the face by sleeping on one side of the face or resting your head in your hands.
Birth control pills, steroids, anti-convulsive medications, and lithium (used to treat some forms of depression).

One Misconception:

Acne is not caused by dirt and cannot be washed away; it is an imbalance (hormonal or chemical) that causes the oil in the skin to form blackheads